My goal is to become a leader in HIV prevention intervention research. K23 award support will allow me to develop the skills I need to become an independent researcher, with a focus on developing and implementing behavioral interventions to prevent HIV. I will acquire training in 1) behavioral science to better understand HIV risk behavior and theories about substance use; 2) qualitative research skills to perform formative research for intervention development; and 3) designing and implementing HIV prevention interventions by answering formative research questions and translating formative research data into a full-scale intervention. I will apply this new knowledge to the study of club-drug using men who have sex with men (MSM). While effective behavioral interventions have been developed for MSM and injection drug user (IDU) populations, less attention has been paid to club-drug use among MSM, and the relationship between club-drug use and high-risk sexual behavior. I hypothesize that 1) club drugs directly contribute to high-risk sexual behavior among MSM; 2) characterizing the size, stability, and sociodemographics of club-drug and sexual networks of MSM is critical in designing interventions to reduce club-drug associated sexual risk behavior; and 3) a network intervention is a feasible and potentially effective way to reduce sexual risk behavior associated with club-drug use. To address the current gaps in club-drug and sexual risk behavior research, the 3 specific aims of my research plan during the K award period are 1) to identify, through quantitative work, which types and combinations of club drugs are associated with high-risk sexual behavior, and to characterize MSM club-drug networks and the sexual networks of club-drug using MSM; 2) to conduct qualitative interviews and collect field observation notes among club-drug using MSM to examine the interpersonal and contextual factors associated with club-drug networks and environments, not measurable through quantitative research; and 3) using results from specific aims 1 and 2, to design and pilot the feasibility and acceptability of a club- drug network intervention to reduce sexual risk behavior associated with club-drug use. Results from these complementary specific aims will be used to secure R01 funding for a larger, phase III randomized trial to determine the efficacy of a network-based club drug intervention to reduce high-risk HIV sexual behaviors.